Why do a slow-moving loaded truck and a speeding rifle bullet have a large momentum?

Momentum is described as "inertia in motion". It is the product of the mass and velocity of an object.

A loaded truck would have a large mass. This would mean that, despite it's small velocity, it has a large momentum.

The speeding rifle bullet has a small mass, but it might be travelling at hundreds of meters per second, which means that it's momentum is still quite high.

Therefore both the bullet and truck have high momentums.